Research Article pubs.acs.org/acscatalysis
Mechanistic Insights into Lewis Acid Metal Salt-Catalyzed Glucose Chemistry in Aqueous Solution Hannah Nguyen, Vladimiros Nikolakis, and Dionisios G. Vlachos* Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, 221 Academy Street, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: While the mechanisms of glucose transformation via heterogeneous metal-substituted BEA zeolites have recently been elucidated, understanding of the same chemistry in homogeneous metal salt catalysts in water is still limited. Here, we investigate the mechanisms of various Lewis acid metal(III) chlorides in glucose isomerization, epimerization, and other interconversions using nuclear magnetic resonance spectroscopy (13C NMR and 1H NMR). We show that the metal chlorides isomerize glucose to fructose via a C2−C1 intramolecular hydride transfer, despite their wide range of catalytic activity. Glucose epimerization to mannose proceeds via two parallel mechanisms, a reverse C2−C1 hydride transfer, and a C1−C2 intramolecular carbon shift (the Bilik reaction), with the hydride transfer being predominant. We hypothesize that the relative activity for the Bilik reaction correlates with the ionic radius of the catalyzing metal aqua cation, with an optimum at ∼1 Å for lanthanide cations. In addition, we show that the metal chlorides, especially chromium(III), facilitate glucose isomerization to sorbose via a C5−C1 hydride transfer, similar to Ti-BEA. These findings bring new insights into Lewis-acid-mediated sugar rearrangements in aqueous solution. KEYWORDS: homogeneous catalysis, metal salts, Lewis acid, glucose, hydride transfer, carbon shift, Bilik reaction, aqueous solution
■
the former catalyst is more active.10 These Lewis acid BEA zeolites also catalyze the conversion of fructose to mannose via a subsequent reverse 1,2 hydride transfer, with a faster rate in methanol than in water.11 Interestingly, when the reaction occurs in the presence of borate salt or when sodium is incorporated into the active site (Na-Sn-BEA), a C2−C1 intramolecular carbon shift, known as the Bilik reaction,12 occurs to directly form mannose as the major product.11,13 Unlike Sn-BEA, the Ti4+ center in Ti-BEA also facilitates a parallel path to convert glucose to sorbose through a C5−C1 hydride transfer (1,5 hydride transfer).9 The different glucose interconversion mechanisms are summarized in Scheme 1. In all cases, it is believed that the active sites around the metal (Sn, Ti) center are partially hydrolyzed, forming the so-called “open site” structure.8,14,15 Another class of Lewis acids effective for the glucose isomerization entails metal halides. Specifically, Zhao and coworkers introduced chromium chloride in ionic liquid to carry out glucose conversion to HMF.16 Subsequently, several other groups implemented one-pot synthesis of HMF from glucose using metal chlorides in different solvents, such as ionic liquids,17,18 organic solvents19,20 and water.4,7,21 The rather unexpected tandem catalysis of Lewis acid isomerization of glucose to fructose, followed by Brønsted acid catalyzed
INTRODUCTION Renewable production of chemicals and fuels from biomass has drawn considerable attention, because of increasing concerns about CO2 emissions. Among the top platform chemicals produced from biomass, 5-hydroxymethyl furfural (HMF) has been a major target for the formation of polymer precursors, fuel additives, and pharmaceuticals.1 HMF can be produced from cellulose via cellulose hydrolysis to glucose, glucose isomerization to fructose, and fructose dehydration to HMF.2 Currently, HMF production is not commercial, partly due to the cost and the equilibrium limitations of the commercial enzymatic glucose isomerization to fructose.3 To overcome these challenges, research has turned to thermochemical catalysts to carry out glucose isomerization. Such catalysts are compatible with Brønsted-acid catalyzed fructose dehydration and can enable integration of the three aforementioned steps in one pot.4−7 A key development in this direction was made by Davis and co-workers, who introduced, for the first time, substituted BEA zeolites (Sn-BEA, Ti-BEA) as effective Lewis acid catalysts for the glucose isomerization to fructose.6−10 Aside from fructose, these Lewis acid BEA zeolites also convert glucose to other monosaccharides, such as mannose and sorbose.9,11 The catalytic activity, selectivity, and mechanism of glucose interconversion vary with catalyst and reaction media. While both Sn-BEA and Ti-BEA isomerize glucose to fructose via a C2−C1 intramolecular hydride transfer (1,2 hydride transfer), © XXXX American Chemical Society
Received: November 27, 2015
1497
DOI: 10.1021/acscatal.5b02698 ACS Catal. 2016, 6, 1497−1504
Research Article
ACS Catalysis
speciation of metal chlorides in aqueous solution identified partially hydrolyzed metal aqua ions (i.e., [CrOH]2+ and [Al(OH)2]+ for CrCl3 and AlCl3, respectively)7,23 and have been hypothesized to be the active species. These species possess a Lewis acid center and OH group(s) as a Brønsted base to cooperatively facilitate the isomerization mechanism, and, as such, they resemble the structure of open sites of SnBEA and Ti-BEA. These analogies open an opportunity to leverage the fundamental knowledge on homogeneous metal salts to better understand and improve heterogeneous Lewis acid catalysts, whose synthesis is often challenging.7,22,24 Despite recent work on metal salts for sugar conversion, mechanistic studies on glucose epimerization by homogeneous Lewis acids is still limited. In this work, we examine, for the first time, the mechanism of mannose formation from glucose in aqueous solution using various metal(III) chlorides. We also report the formation of hexoses other than fructose and mannose from glucose. We show that metal chlorides not only display a wide range of catalytic activity but also have different selectivity. Our study further delineates the mechanistic similarities and differences between homogeneous and heterogeneous Lewis acid catalysts.
Scheme 1. Summary of the Mechanism of Glucose Interconversions by Lewis Acids Known to Date
dehydration of fructose to HMF in the absence of external acid was rationalized by realizing that metal salt hydrolysis produces Brønsted acidity.7 Our mechanistic studies of glucose isomerization by CrCl3 and AlCl3 showed striking analogy to the mechanism of glucose isomerization to fructose by Sn-BEA.22 In all cases, the reaction proceeds through 1,2 hydride transfer and exhibits a kinetic isotope effect, indicating that the hydride transfer is the rate-limiting step.10,22 Studies on the kinetics and
■
EXPERIMENTAL METHODS The catalysts, including the hydrated form of CrCl3, AlCl3, InCl3, GaCl3, LaCl3, DyCl3, and YbCl3, were purchased from Sigma−Aldrich and used without further modification. A typical kinetic experiment was carried out in a 5 mL glass vial
Figure 1. (Bottom) 13C NMR spectra of (a) a standard mixture of glucose and fructose, (b) a standard mixture of D2 glucose and fructose, and (c) sugar fraction after reaction in GaCl3. (Left blue box at top) Magnification of the spectra within the C2 glucose region bracketed with blue dashed lines. The two C2 glucose peaks are labeled with vertical blue dashes lines. (Right green box at top) Magnification of the spectra within the C1 fructose region bracketed with green dashed lines. The three C1 fructose peaks are labeled with vertical green dashed lines. 1498
DOI: 10.1021/acscatal.5b02698 ACS Catal. 2016, 6, 1497−1504
Research Article
ACS Catalysis
Figure 2. (Left) 13C NMR of (a) a standard mixture of 13C−C1 glucose, fructose, and mannose (spectrum a), and the sugar fraction after reaction using NaOH (spectrum b), YbCl3 (spectrum c), and CrCl3 (spectrum d). (Right) Magnification of the spectra of the C1 mannose peaks (in green), the C2 mannose peaks (in blue), and the C1 fructose peaks (in red).
■
containing 2 mL of aqueous solution of glucose and a metal(III) chloride catalyst (MCl3). The reactors were heated in a temperature controlled aluminum block, consisting of different oil wells, on a stirring hot plate. After a certain time, the glass vials were quenched immediately in an ice bath and the post-reaction solutions were filtered and analyzed using high-performance liquid chromatography (HPLC) consisting of a Waters Alliance System (an e2695 HPLC), with a refractive index (RI) detector, photodiode array (PDA) detector, and a fraction collector. Post-reaction mixtures were analyzed and separated using a Biorad HPX 87C column or a Biorad HPX 87P column. HPLC grade water was flowed at a rate of 0.5 mL/ min as a mobile phase; the column temperature was 75 °C, and the RI detector temperature was 35 °C. Calibration curves to correlate the integrated peaks with the compounds’ concentration were prepared from standard solutions. The carbon loss due to humins formation is
